Turbofan aircraft engines have large fans at the forward end. They rotate at a high speed at about 4000 revolution per minute.
Foreign objects such as birds, hailstones or debris ingested from the ground occasionally strike the fan blades. It is possible for this to cause the fan blades to fragment. These fragments may be on the order of 7 kilograms traveling at about 930 meters per second. It is essential to contain the blade fragments and also to retain the casing.
A typical containment structure is shown in U.S. Pat. No. 4,490,092 entitled "Containment Structure" and issued to Emile J. Premont. A support structure has "c" shaped stiffeners between inner and outer sheets. This structure surrounds the fan and has multiple layers of woven KEVLAR.RTM. ballistic fabric (trademark of DuPont Corporation). This fabric is wound under tension and serves to resiliently contain blade fragments passing through the support structure.
FIGS. 10 through 15 of the above-cited U.S. Pat. No. 4,490,092 illustrate the general track of a blade fragment passing through the support structure and retained by the ballistic fabric. The blade fragment has an aft component and moves in the aft direction pulling the ballistic fabric with it. The ballistic fabric is pulled downstream with the fabric from the forward location coveting the opening. The fabric on occasion is pulled into the hole by the rotor during this failure event. Interaction of the fabric with the blades causes additional damage.
This has been avoided in the past by making a large honeycomb structure positioning the cloth well away from the rotor. This however increases the diameter required. An alternate approach would be to use mechanical fasteners to keep the fabric in place, but this could lead to concentrated loading and tearing of the fabric.